Kaolin or kaolinitic clay is a clay containing rock comprised of kaolin minerals that is white or nearly white, or can be beneficiated to be white or nearly white. Such clays were formed by the weathering or hydrothermal alteration of the feldspar and mica mineral components contained in feldspathic rocks such as granite and gneiss. Primary kaolin clays are those found in deposits at the site at which they were formed, and they generally contain with the kaolin minerals some unaltered mica and possibly feldspar, and stable minerals such as quartz, which must be separated from the clay during the refining process for the clay products. For example, primary kaolin deposits are mined and processed in places such as southwest England, France, Germany, Spain, and the Czech Republic. Sedimentary kaolin, which are alternatively known as secondary kaolin, are those which contain sediments eroded from a feldspathic source rock and deposited in fluvial or marginal marine environments such as those associated with deltas. For example kaolin clays mined and processed from deposits in Georgia, South Carolina, and Alabama, in the United States, and from deposits in Para state of Brazil are generally of the sedimentary type. Sedimentary clays deposited in different geological eras can also be used, such as tertiary or cretaceous clays.
Rare earth elements include the fifteen lanthanides, scandium, and yttrium (i.e., corresponding to the elements associated with atomic numbers 57-71, 21, and 39, respectively). Rare earth elements, although relatively plentiful in the earth's crust, are typically dispersed and not found in concentrated quantities or economically exploitable forms in most crustal rocks. Exceptions include igneous rocks such as the carbonatites located at Mountain Pass, Calif.; peralkaline syenites and granites located at Pajarito Mountain, N. Mex.; and monazite-apatite veins hosted in granite near Crescent Peak, Nev. The largest rare earth deposit in the World is at Bayan Obo in Inner Mongolia, China. The rare earth at Bayan Obo is recovered as a byproduct of iron production from a variety of magnetite-bearing rock types. Rare earth elements are concentrated in specific accessory minerals associated with the ore. Some of the minerals that can contain rare earth elements are apatite, bastnasite, florencite, monazite, xenotime. Bastnasite is a lanthanide carbonate. Apatite, monazite, and xenotime are phosphates. Florencite and a variety of lesser known rare earth containing minerals are hydrated aluminum phosphates. The phosphate-bearing minerals resist chemical attack from weathering, and they can be found in sedimentary deposits. Some marine heavy mineral sand deposits that contain monazite and xenotime which can be economically separated are at Eneabba, Australia; and other heavy mineral beach sand deposits mined and processed as byproducts in Brazil, India, Malaysia, Thailand, China, Taiwan, United States, New Zealand, Sri Lanka, Indonesia, Zaire, and Korea. Some kaolinitic claystone (sedimentary kaolin) deposits contain trace concentrations of hydrated aluminum phosphates, but no commercially developed sedimentary kaolin deposits are known to produce rare earth concentrates.
The processes used for separation of minerals containing rare earth elements from deposits is specific to each occurrence. The surface or underground mining of a rare earth ore from hard rock such as a carbonatite or granite typically requires mineral liberation by crushing and grinding the ore prior to beneficiation. Flotation is used at Mountain Pass, Calif. to concentrate liberated bastnasite particles from an ore containing about 5 wt. % REO (rare earth oxide) to a form a concentrate containing about 60 wt. % REO. Placer sand deposits containing grains of monazite and xenotime may use gravity separation equipment such as jigs, spiral classifiers, and hydrocyclones to separate discrete sand particles of heavy minerals from light minerals based on their different specific gravity. The heavy minerals from gravity separation are then fed to a magnetic separator to remove magnetic minerals, an electrostatic separator to remove titania minerals, and a final magnetic separation state to separate zircon. The rare earth mineral concentrate produced from a variety of geological environments is then fed to a process that extracts the REE (rare earth elements) by dissolution in hot concentrated acid or alkaline leaching. Further chemical fractionation to separate specific rare earth elements occurs after the leaching stage.
Rare earth elements are a valuable commodity due to their numerous uses in modern devices and processes. For example, rare earth elements are often used in batteries, compact fluorescent lights, aerospace components, flat panel displays, high-temperature superconductors, lasers, polishing compounds, catalysts, water purification devices, compact speakers, and electronic devices, among other devices and methods. Thus, although concentrated deposits of rare earth elements are rare, the demand for rare earth elements is increasing. As a result, it would be desirable to identify new economically-viable ways to concentrate useful quantities of rare earth elements from sources in which the rare earth elements are disseminated in REO-bearing minerals having a low bulk concentration in the rock. For example, because rare earth elements may be found as a component in minerals disseminated at low bulk concentrations in rocks having high concentrations of kaolinite, it would be desirable to develop an economically-viable way to separate rare earth element compositions from material containing kaolinite, such as kaolin.
In addition to rare earth element compositions, kaolin also often includes a number of substances considered to be impurities, particularly when the kaolin is refined for use as an ingredient, pigment, or filler material in a variety of application compositions, such as, for example, filling and coating of paper, paper board, and similar products. For example, kaolinite is a white mineral and is often used in such application compositions to impart, amongst other things, whiteness and brightness. However, one or more desirable properties of the kaolin may be adversely affected by the presence of impurities. Typical impurities may include, but are not limited to, titania-bearing minerals such as anatase and rutile, quartz, mica, phosphates, smectite, and other silicate, oxide and hydroxide compounds containing transition elements such as iron and chromium, and heavy metals such as lead and thorium. Thus, it would be desirable to develop a method of treating kaolin or other kaolinite-containing compositions in such a way that not only are rare earth element compositions separated from the material containing kaolinite, but other impurities are also separated from the kaolin or kaolinite-containing compositions.